The present invention relates to an improvement in a signal copy device for a digital record and/or reproduction system for copying digital information signals.
With the analog record/reproduction devices such as audio tape recorders, improvements in the S/N ratio, the dynamic range and the frequency characteristics largely depend upon improvements in the performance of the record/reproduction head and the drive mechanism for the recording medium. However, most of the technically feasible improvements in the drive mechanism, that is, elimination of wow fluttering and speed deviation have already been made. Many engineers have asserted that further improvements in the performance of the drive mechanism are unattainable at the present moment. Therefore, great improvements in the characteristics of the analog record/reproduction devices are not expected.
In order to overcome the technical limits of the analog record/reproduction devices, digital record/reproduction systems have recently been developed. With these digital record/reproduction systems, analog data (e.g., audio signals) are sampled, quantized and converted into a digital encoded signals of predetermined bit length. The encoded signals are recorded on a recording medium. By detecting if the respective pulse components of the digital encoded signals are 0 or 1 for reproduction, the waveform of the original analog information signal may be reproduced with high fidelty. In this manner, the reproduction performance such as the S/N ratio, the dynamic range and the frequency characteristics may be easily improved over the conventional systems, independently of the performance of the drive mechanism of the recording medium or the like. This is advantageous in that the signal quality is not degraded and the signal is not lost after repeated copy operations of the information signal.
In a conventional digital record/reproduction system, the output end of an encoding circuit for converting an analog information signal into a digital encoded signal is connected to a first digital signal processing circuit through a switch section. The signal processing circuit serves to add an error correction signal and an error detection signal to the digital encoded information signal. The output end of the first signal processing circuit is connected to a video tape recorder (VTR). The output end of the VTR is connected to the input end of a second digital signal processing circuit through a single connection terminal. The second signal processing circuit serves to correct the error of the information signal. The output end of the second signal processing circuit is connected to the switch section and to a decoding circuit. The switching section performs a switching operation to selectively supply the first signal processing circuit with output signals of the encoding circuit and the second signal processing circuit.
In order to copy to the VTR (first VTR) the digital encoded information signal which is recorded in advance on another VTR, the digital encoded information signal to be transferred must be supplied to the second signal processing circuit. Therefore, with a conventional digital record/reproduction system, for changing from the normal record mode to the copy mode, the output end of the first VTR must be disconnected from the second connection terminal and the output end of the second VTR (storing the information signal to be copied) must be connected to the connection terminal. This is because there is only one connection terminal which is connected to the second signal processing circuit. For this reason, every time the operation mode is changed, the connection of the first and second VTRs to the second signal processing circuit must be changed, resulting in inconvenience.
When the switch section is erroneously operated under the condition that the first VTR is connected between the output end of the first signal processing circuit and the input end of the second signal processing circuit, the output end of the second signal processing circuit and the input end of the first signal processing circuit are directly coupled to each other. When this happens, the first VTR and the first and second digital signal processing circuits form a closed loop. Even if an abnormal signal is generated within this closed loop and is superposed on the normal information signal, the correction of the abnormal signal cannot be performed by the second signal processing circuit. If the upper significant bit of the data constituting the encoded signal changes from logic value "0" to logic value "1" due to the superposition of the abnormal signal, the analog signal obtained by decoding this encoded signal becomes a signal of extremely great amplitude. In the worst case, this may damage other system equipment (e.g, loudspeakers) connected to the output section. Furthermore, the closed loop may cause undesirable oscillation. Consequently, the reliability of the system is vastly degraded.